Re: Wheel size revisited

[Kevin Wenzel <kwenzel@privateI.com>]

On Mon, 22 Dec 1997, Paul Krucke wrote:

> ---Iain Mannix  wrote:

> > Why is this worse for handling?  Not saying it is, not saying it
> is'nt -
> > just curious.
> 
>    You narrow your track sir....  Wider is better--no?  Generally
> speaking you want to move your wheels to the outside rather than the
> inside.  

Actually, assuming that you've succeeded in fitting a wider wheel, it's
a really good idea to gain as much backspacing (4.5-5" on our cars is
about the most you can get) as you can without rubbing significantly
against the strut/spring assembly.  Keeping the center of the tire closer
to the inside of the car is desirable because it keeps the scrub radius
negative.  Bad ASCII art (will only make sense with a mono-spaced font
like Courier) follows:

\
 \< Kingpin inclination (line drawn from top of strut through
  \                      ball joint - not necessarily the angle
   \                     of the strut itself)
    \
     \       < centerline of tire (sort of)
      \  ___|__
       \|      |
        \      |
        |\     |
        | \    |    >>>outside of car
  ------|--\   |
        |   \  |
        |____\_|
            |-| < the "-" represents the difference between the center
                  of the contact patch and the spot where the kingpin
                  angle contacts the ground - that's the scrub angle.


The above, uh, picture :) shows negative scrub angle - this is *very*
desirable on the front suspension of a FWD car.  If you use spacers,
or a wheel with very little backspacing (say, 2-3") you end up with
zero or positive scrub angle - this will tend to make the car push a
lot and handle pretty badly in transition.  On the street you wouldn't
notice much except that it won't seem to want to change direction as
quickly when pressed hard.

In other words, you really want the inside of the wheel as far in you
can get it (at least for autocross cars - that's all I have any
experience with at the moment).  This is a lot easier to deal with
on race wheels - 13x9 wheels with 5" of backspace won't rub at all,
but I imagine you might have problems on the street with larger
diameter wheels.

(Note for autocrossers: that kingpin angle is the cause of camber loss
with steering lock - you must add as much caster as you can to correct
for it.  With a significant kingpin angle (like all FWD cars) and 
little or no caster, turning the wheels will result in positive camber
(bad) on the outside wheel - caster is the only way, short of relocating
suspension mounting points, to get rid of this)

Kevin Wenzel
1981 VW Scirocco S (w/1.8 JH) - DSP #81 (Blix)
Director of Network Operations, privateI, LLC        
http://www.privateI.com/

--
To subscribe or unsubscribe, send email to scirocco-L-request@privateI.com,
with your request (subscribe, unsubscribe) in the BODY of the message.